The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina (Al2O3) layer characterized by fine, equiaxed grains.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Cemented carbide cutting tools can be coated with various types of Al2O3 layers by using Chemical Vapour Deposition (CVD), e.g., pure xcexa-Al2O3, mixtures of xcexa- and xcex1-Al2O3, coarse grained xcex1-Al2O3, and fine grained textured xcex1-Al2O3 have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.
Al2O3 crystallizes in several different phases: xcex1, xcexa, xcex3, xcex4, xcex8 etc. The two most frequently occurring phases of CVD-produced wear resistant Al2O3 layers are the thermodynamically stable xcex1-phase and the metastable xcexa-phase, or a mixture thereof. Generally, the xcexa-phase exhibits a grainsize in the range 0.5-3.0 xcexcm and the grains predominately grow through the whole coating forming a columnar type coating morphology. Furthermore, the xcexa-Al2O3 layers are free from crystallographic defects and also free from micropores and voids.
Coarse-grained (3-6 xcexcm) xcex1-Al2O3 often possesses porosity and crystallographic defects, while fine-grained textured xcex1-Al2O3 are free of defects with very pronounced columnar-shaped grains.
In U.S. Pat. No. 5,674,564 is disclosed a method of growing a fine-grained xcexa-alumina layer by employing a low deposition temperature and a high concentration of a sulphur dopant.
In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a fine grained, (012)-textured xcex1-Al2O3 layer consisting of columnar grains with a small cross section (about 1 xcexcm).
In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a fine-grained (104)-textured xcex1-Al2O3 layer.
As mentioned above, all Al2O3 layers produced by the CVD technique possess a more or less columnar-like grainstructure. An Al2O3 layer with an equiaxed grainstructure is, however, expected to show some favorable mechanical properties, e.g.xe2x80x94resistance to crack propagation, as compared to a layer with a columnar grainstructure. One well-known and possible technique to avoid columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of Al2O3 is periodically interrupted by the growth of a thin, 0.1-1 xcexcm second layer such as disclosed in U.S. Pat. No. 4,984,940. The second layer should preferably have a different crystal structure or at least different lattice spacings in order to be able to initiate renucleation of the first layer. One example of such a technique is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in a (Al2O3+TiN)xn multilayer structure with a thickness of the individual TiN layers of about 0.1-1 xcexcm (see, e.g.xe2x80x94Proceedings of the 12th European CVD Conference, page pr.8-349). However such multilayer structures very often suffer from a low adherence between the two different types of layers.
It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiCxNyOz layer, at least one single phase xcex1-Al2O3 layer with a microstructure which is different from the prior art columnar xcex1- or xcexa-Al2O3 CVD layers mentioned above. It is also the object of the present invention to provide a high performance tool coating comprising the invented Al2O3 layer.
It is a further object of the invention to provide an alumina coated cutting tool insert with improved cutting performance in steel, stainless steel, cast iron and in particular nodular cast iron.
According to one aspect, the present invention provides a cutting tool comprising a body of sintered cemented carbide, cermet, or ceramic superhard material, the body comprising a surface, and a hard and wear resistant coating on at least a portion of the surface, said coating comprising: one or more refractory layers of which at least one layer essentially consists of xcex1-Al2O3, said xcex1-Al2O3 layer having equiaxed grains with an average grainsize of  less than 1 xcexcm and further containing striated zones containing  greater than 5 at % titanium, but no nitrogen or carbon.
According to another aspect, the present invention provides a method of coating a body with an xcex1-alumina layer comprising: (i) bringing the body into contact with a hydrogen carrier gas containing one or more halides of aluminum and a hydrolyzing and/or oxidizing agent while the body is at a temperature of 950-1000xc2x0 C.; (ii) maintaining the oxidation potential of the CVD-reactor atmosphere prior to the nucleation of Al2O3 at a low level, using a total predetermined concentration of oxidizing species; (iii) starting Al2O3 growth by introducing the following gases into the reaction chamber: AlCl3, HCl and CO2; (iv) adding a sulphur dopant after 20-60 min; (v) repeatedly stopping the CO2, AlCl3, HCl and the sulphur dopant for intervals of 10-50 min during which TiCl4 is allowed to enter the reactor for 1-10 min in a concentration of 1-10%; and (vi) then reintroducing AlCl3, HCl, CO2 and the sulphur dopant, in that order.